南天山榆树沟麻粒岩地体的尖晶石研究

A Study of Spinels from a Granulite Terrain in Yushugou, South Tianshan Mountain

新疆榆树沟麻粒岩地体中发育两类尖晶石：一类见于中、基性麻粒岩体中，其化学成分富 Ａｌ 贫 Ｃｒ ，属铝尖晶石，镜下为深绿色，呈半自形至他形粒状，与其它变质矿物共生，表现为麻粒岩相的新生变质矿物；另一类发育在空间上与麻粒岩体紧密相邻的超镁铁质岩体中，为铬尖晶石，镜下为深褐红色，呈不规则粒状分布于橄榄石、斜方辉石和单斜辉石之间，为麻粒岩相变质过程中稳定的残余矿物。后者属于 Ｄｉｃｋ 等人划分的 Ｉ 型尖晶石，它的存在说明超镁铁质岩体为大洋岩石圈地幔的组成部分。这两类尖晶石的特征一方面说明该地体遭受了麻粒岩相的变质作用改造，另一方面也提供了榆树沟麻粒岩相蛇绿岩套属洋盆构造环境的直接矿物学证据。

There are two kinds of spinels in a granulite terrain of Yushujgou, South Tianshan Mountain. One occurs in the intermediate and basic granulite units and is characterized by enriched Al and poor Cr, belonging to Al_spinel. In thin sections it shows dark green color and subhedral to anhedral form, and coexists with other metamorphic minerals, suggesting a neogenic metamorphic mineral of granulite facies. The other occurs in the adjacent ultramafic rock unit, belonging to Cr_spinel. In thin sections, it is dark brown in color and is scattered among olivine, clinopyroxene and orthopyroxene as irregular grains, indicating a stable residual mineral in the process of granulite facies metamorphism. The spinel in the ultramafic rock unit belongs to I_type spinel, as proposed by Dick et al . This implies that the ultramafic rock is a part of oceanic lithosphere mantle. Characteristics of these two kinds of spinels not only indicate that the whole terrain underwent granulite facies metamorphism but also provide direct mineralogical evidence for the argument that Yushugou ophiolite suite of granulite facies lies in an environment of oceanic basin.